does test PSI matter in a leak down test?

[bumperbozo]

Here are the instructions for my cylinder leakage tester:



It basically indicates that you can use any test PSI under 100 and looking at the table it appears that the leakage is calculated as the difference between the gauges divided by the test PSI. But I can imagine that if you have a pretty low test PSI, the impact of the tool orifice vs the opening(s) in the cylinder is going to change. Like, a shake straw doesn't restrict your blowing if you're not blowing that hard.

Do the right and left gauges scale linearly for any given amount of ring gap, for instance?

The reason I'm asking is that the leak down test the dealer tech did on the car we are looking at had a different test PSI for each cylinder so I want to make sure they are comparable.

Incidentally, using the above math, the cylinders had 1.1%, 0.0%, 2.9%, 0.6%, 2.4%, and 0.0% leakage, respectively. The engine has 15k miles and it is a honda J35Z8 V6. Is this a plausible or likely result?

 

[Neggy]

a leak down test for a honda with 15K miles?

kind of a waste of time, unless it was a former rental.

there are 50 things I'd be looking at before I did that

I would have done a starter draw test and watch the draw as it spun looking for a weak cylinder

those numbers tell part of story... no compression test?

 

[bumperbozo]

a leak down test for a honda with 15K miles?

kind of a waste of time, unless it was a former rental.

as i've explained to others, not when its a $70k custom modified wheelchair van with some question marks in it's carfax.

there are 50 things I'd be looking at before I did that

I had already looked at 100 things prior do doing this and, well, it's already done.

I would have done a starter draw test and watch the draw as it spun looking for a weak cylinder

yes, this is a great way to screen before committing to pulling plugs.

those numbers tell part of story... no compression test?

Cylinder 1 at 175-176
Cylinder 2 at 181-184
Cylinder 3 at 180-185
Cylinder 4 at 180
Cylinder 5 at 184-185
Cylinder 6 at 184-185

(the tech took pictures of the gauge from different angles, some of which made the needle look like it was in a slightly different position than it was actually likely in. that's why some of them are stated as ranges.)

 

[merkyworks]

Dont know specs of your Honda engine but when doing a leak down test you want to see consistent numbers across all cylinders.

From #’s you provided there’s roughly a 5% difference which is pretty good in my book. I would say things look good but I’m not a Professional Mechanic.

 

[Neggy]

so if we take 180 as the norm, you are +/-4 at most and that is with questionable pictures, I'm not sweating those numbers

Compression gives you the condition of the engine, leak down is a tool to help figure out component is bad if the compression test is off... intake valve, exhaust valve, rings, head gasket

What is questionable on the car fax?

The first think I ask when I see a low mileage near new car for sale is WHY it is for sale.

The second think I do is get the VIN and call a friend at a dealer and get the warranty and service history for the vehicle.

CarFax is not to be taken a the end all be all , I have fixed dozens of totaled cars, including back halfing a ZR-1 that there was no police report or insurance claim filed

Ask to see the title, snap a pic, call the previous owner.

if the back of the title shows it has been moved around the country, or from dealer to dealer ask yourself why.

A specialty vehicle like that is normally used by the original owner for a long time, it is unusual to see current model low mileage ones out there

Accident damage?

Lemon law buyback?

 

[bumperbozo]

so if we take 180 as the norm, you are +/-4 at most and that is with questionable pictures, I'm not sweating those numbers

Compression gives you the condition of the engine, leak down is a tool to help figure out component is bad if the compression test is off... intake valve, exhaust valve, rings, head gasket

What is questionable on the car fax?

The first think I ask when I see a low mileage near new car for sale is WHY it is for sale.

The second think I do is get the VIN and call a friend at a dealer and get the warranty and service history for the vehicle.

CarFax is not to be taken a the end all be all , I have fixed dozens of totaled cars, including back halfing a ZR-1 that there was no police report or insurance claim filed

Ask to see the title, snap a pic, call the previous owner.

if the back of the title shows it has been moved around the country, or from dealer to dealer ask yourself why.

yep, looked into all those things. questionable because it had never had it's oil changed over 6 years, it lived in a dusty area and was used infrequently, and other things. plus, the buyer only gets one shot at using her fundraising money to buy a car. hence, me being completely thorough with the due diligence.

A specialty vehicle like that is normally used by the original owner for a long time, it is unusual to see current model low mileage ones out there

Accident damage?

Lemon law buyback?

my guess is the original person for whom the vehicle was purchased died. disability and death tend to go hand in hand. body inspection looked pretty good, but it can be hard to tell because the wheelchair modification process involves a lot of removing panels, welding, half resprays on doors, disassembling suspension, etc.

 

[Nthill93]

How do you know it never had an oil change in 6 years? If that’s true why are you even wasting your time looking at it. That’s a hard pass unless you’re going an unreal deal

 

[Neggy]

unless you were getting it for short money, like NADA clean trade, (assuming 70K miles) in value with a little something for the lift, I'd run

Cars that are barely used , unless stored indoors with protection from the elements turn into nightmares, For example my former 2005 F150 with 63K on it had a frame failure from being parked outside and rarely being used, my 2007 Mustang with 9000 miles on it has spent the last 10 years of its life in climate controlled garages and is perfect. All the factory tags are still on the parts from when it was made.

Sitting is really bad for cars... you know it needs tires, brake flush, transmission and coolant service, and I'd be using a litmus test on the coolant to see if it is non acidic because if the coolant is **** that can get expensive. I'd check the brake fluid for water content with a tester

You don't have a factory warranty to fall back on, and as you alluded to you are at the mercy of the upfitter WRT corrosion protections that were used when it was cut up for the lift... which brings us to the lift can you get parts for it if you had to?

After reading that it is 6 years old with low mileage and no documented service history I'd be walking on eggshells too

 

[signcrafter]

So because carfax doesnt show any oil changes in 6 years that means they never changed the oil? Not all places report to carfax. If you check carfax my vehicles havent had the oil changed since I got them since I do it myself.

Is there an actual problem with the vehicle that you are trying to diagnose?

 

[Showkey]

Modern car V6 with low compression on one or more cylinders……..will throw a CEL a with mis fire code. Even a slightly tight valve …….mis fire code.

15k. miles….waste for time and money. Take it for a long test drive.

 

[67King]

Oh, lord, Carfax is HORRIBLE. Been burned buying and selling. Buying, bought a "clean" Ram 2500, 4 years old (2007 in 2011), albeit wholesale, that had had tons of bodywork (yes, I know.....but a manual transmission diesel SLT is what I was after, and they are rare). Selling, my wife hit a small deer that took out the radiator because that's just where the bulk of it happened to hit. So it was towed. Carfax put it essentially in the same class as "totaled," justifying it by saying any vehicle that has to be towed has had serious damage. And heaven forbid if one does his own maintenance, which isn't captured.

 

[bumperbozo]

Modern car V6 with low compression on one or more cylinders……..will throw a CEL a with mis fire code. Even a slightly tight valve …….mis fire code.

15k. miles….waste for time and money. Take it for a long test drive.

many people say this, but i've seen no data to back it up, and i've heard anecdotes to the contrary. what kind of compression loss shows up as a misfire? 30%? if i'm down 15% on a car i'm buying i still want to know about it. negotiate on the price or be able to know how much i'm willing to invest in reconditioning.

 

[bumperbozo]

So because carfax doesnt show any oil changes in 6 years that means they never changed the oil? Not all places report to carfax. If you check carfax my vehicles havent had the oil changed since I got them since I do it myself.

Is there an actual problem with the vehicle that you are trying to diagnose?

right, carfax is far from a complete maintenance history. but i tried to call around and reconstruct more records, so the next step was to test the actual engine. no actual problem has been identified, just doing comprehensive due diligence.

 

[bumperbozo]

unless you were getting it for short money, like NADA clean trade, (assuming 70K miles) in value with a little something for the lift, I'd run

Cars that are barely used , unless stored indoors with protection from the elements turn into nightmares, For example my former 2005 F150 with 63K on it had a frame failure from being parked outside and rarely being used, my 2007 Mustang with 9000 miles on it has spent the last 10 years of its life in climate controlled garages and is perfect. All the factory tags are still on the parts from when it was made.

Sitting is really bad for cars... you know it needs tires, brake flush, transmission and coolant service, and I'd be using a litmus test on the coolant to see if it is non acidic because if the coolant is **** that can get expensive. I'd check the brake fluid for water content with a tester

You don't have a factory warranty to fall back on, and as you alluded to you are at the mercy of the upfitter WRT corrosion protections that were used when it was cut up for the lift... which brings us to the lift can you get parts for it if you had to?

After reading that it is 6 years old with low mileage and no documented service history I'd be walking on eggshells too

except used wheelchair vans generally have these kinds of issues. and vans that meet one's own particular needs are far and few between. it's just a much more challenging market to be shopping in. plus with the VCM on these engines, there's kind of a big tradeoff to adding a lot of miles as the rings will often get stuck due to the suction in the inactive cylinders pulling oil past the oil control ring. planning on doing some kind of VCM delete kit.

 

[FuzzyTiger]

Just some recent experience I had troubleshooting an engine with a misfire. Fuel, spark, timing etc all checked out. It was the compression and leak down tests that identified the issue:

- Compression test results:
1-4: 163, 163, 167, 167
5-8: 179, 169, 170, 170

- Leak down results:
1-4: 5%, 6%, 5%, 5%
5-8: 5%, 4%, 2%, 2%

Bonus points for anyone that can figure out which cylinder was misfiring and what the cause was. The engine was a bmw v8 n62.

Spoiler: Answer

Cylinder 5 misfiring due to one of it's intake valves not opening because of a spring in the valve train popping out of position so the rocker arm wasn't engaging. The slightly higher compression on the compression test was the only test that suggested at the cause. Though by normal compression test conventions it is still within the acceptable range.

Another example on the same engine - 0 compression and 100% leakage on all cylinders out the intake. Was the engine mechanically dead? Nope. Just a bad ground caused the variocam system to hold the intake valves open.

The point of my post is just to say that a test on its own doesn't provide any conclusions. Context is what helps you understand whether a test result is good or bad. Your test numbers all look fine. Cylinder 1 seems to be the outlier but in the absence of anything else suggesting a problem like in my case its within an acceptable range.

 

[Showkey]

many people say this, but i've seen no data to back it up, and i've heard anecdotes to the contrary. what kind of compression loss shows up as a misfire? 30%? if i'm down 15% on a car i'm buying i still want to know about it. negotiate on the price or be able to know how much i'm willing to invest in reconditioning.

OBD II has three different levels of severity concerning engine misfires. Depending on the severity, the misfire will be designated as type A, B or C. Each of these misfire types is determined by a percentage of misfires that occur within a specified timeframe and result in specific failure conditions.

link………

StackPath

Live data stream on a quality scan tool will display misfire counts.

 

[u118224]

Why do a leak down if the compression numbers are all good?

 

[Packard V8]

Buying a used vehicle out of warranty is never going to be as assured as our OP would like it. The good news is all of the leakdown and compression numbers are as good as one could hope to find.

And yes, CarFax is a joke. Their computer program hoovers up all the free data in the auto dumping ground, misses a large percentage of any given vehicle's history which isn't out there for free and then applies some arbitrary algorithms to that data. Barely better than nothing and certainly not as good as an experienced and/or professional inspection.

As to the CarFax "serious damage"; a substantial extended family of former-USSR immigrants here make a profitable business from inspecting cars at auction with total and serious damage titles/reports. Many have only cosmetic damage and are easily repairable if an insurance company is not paying the bill at a collision repair shop. They buy and repair them and sell them to others in the immigrant community. Everyone benefits.

jack vines

 

[bumperbozo]

And yes, CarFax is a joke. Their computer program hoovers up all the free data in the auto dumping ground, misses a large percentage of any given vehicle's history which isn't out there for free and then applies some arbitrary algorithms to that data. Barely better than nothing and certainly not as good as an experienced and/or professional inspection.

that's why i'm trying to get good at the latter.

 

[Wrench97]

IF you are just checking for pre purchase and not a actual issue, a oil sample sent to a lab is a lot more definitive then a leak down test.
As long as the oil is not freshly changed.

 

[joecon]

I would question the Cylinder Leakage test because, I have never have gotten a 0% leakage and do not think it is the right reading.
All engines have some leakage. I would not trust any reading below about 5%.

 

[2ndGearRubber]

^ 2% and less isn't uncommon.


VCM delete it for long term use. IDK what this is telling us about the health of the rings/valves on an engine with 15k. A pressure transducer or pulse sensor in the dipstick tube would have been faster, sync against a COP coil. Or just a relative compression test, all relatively even, move on. Even if it never got an oil change from new, it'll live. Pop the oil cap off and take a peek.

If these vehicles fitted in the way you need are so hard to find, just buy it. People beat those vans to death running 10k intervals 3 quarts low for the entire life of the van. Sounds as though even IF engine replacement were needed at some point, it would be a small cost relative to the value of the vehicle, due to the rarity of the added equipment.

 

[67King]

IF you are just checking for pre purchase and not a actual issue, a oil sample sent to a lab is a lot more definitive then a leak down test.
As long as the oil is not freshly changed.

I was going to suggest an oil analysis to look for wear metals. If the oil is acidic, it may just be old. But if it has not had proper maintenance, it should show up in the wear metals. My company (NA importer/distributor for Millers Oils) always recommended this to our racing customers. Ended up going mostly with Blackstone.

 

[FMB4]

Op's question: "does test PSI matter in a leak down test?
The chart shown in the Operating Instructions show Left Hand Gauge pressures of 100, 90, and 75 psi. I wouldn't worry about psi as long as the LD was performed within these pressures.

Op question: Do the right and left gauges scale linearly for any given amount of ring gap, for instance?
I'm not understanding your question. The left gauge indicates the compressor's 'input' pressure. The right gauge indicates the leak down pressure drop due to ring gap, cyl wear, valve sealing, and head gasket, etc, conditions.

Good luck, and I hope you find a solid vehicle that suits your needs!

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